Article pubs.acs.org/acssensors
Hybrid Sensor Based on AlGaN/GaN Molecular Controlled Device Meital Eckshtain-Levi,† Eyal Capua,† Yossi Paltiel,‡ and Ron Naaman*,† †
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel Applied Physics Department and the Center for Nano Science and Nanotechnology, The Hebrew University, Jerusalem 91904, Israel
‡
S Supporting Information *
ABSTRACT: We developed and investigated the properties of molecularly controlled semiconductor resistors (MOCSERs) based on AlGaN/GaN structure. The response of the sensor for two different analytes was investigated when the sensor was coated with two molecules that differ only in their binding groups. We studied the ability to enhance the specificity of the sensor by adding illumination at various sub-bandgap frequencies. It was verified that the sensor is sensitive to the electronegativity of the analyte and illumination can affect the sensitivity and selectivity when the system does not reach a steady state. Hence, we differ between two operational modes in which orthogonal sensing is made.
KEYWORDS: MOCSER, AlGaN/GaN, side gates, sub-bandgap excitation, ammonia, formic acid
A
The operation of a MOCSER device depends on surface treatments and surface modification by molecular adsorbates. The adsorbed layer can provide a route to enhance chemical stability and improved sensor selectivity and sensitivity. Several studies demonstrated covalent and noncovalent methods for coating GaN surfaces. Adsorption by the covalent approach is usually achieved on the hydroxylated GaN surface.23−27 Thiolated molecules, on the other hand, may be adsorbed directly on the oxide-free surfaces.28,29 The introduction of selfassembled monolayers may produce a net surface dipole, which leads to modification of the surface workfunction and of the band bending.30−33 In principle it is expected that a GaN based device will be sensitive solely to the surface electrochemical potentials (as seen in Figure 1b,c) and blind to sub-bandgap illuminations, namely, light radiation above 365 nm. However, recently it was observed that a GaN based device may respond to visible light.28 In the current study, we probed whether the sensing properties of a GaN based MOCSER can be improved by combining the chemical sensing with illumination at the subbandgap wavelength. We show that by using GaN sub-bandgap illumination, additional molecular signatures could be probed to identify an analyte using the same sensor. The MOCSER sensors were built based on AlGaN/GaN HEMT devices and their response was monitored when exposed to vapors of two molecules, ammonium hydroxide and formic acid, and under illumination by different wavelengths. The common MOCSER structure was modified by adding side gates. The side gates
pplications based on wide bandgap semiconductors have rapidly matured in recent years due to the ability to apply these semiconductors in high-power, high-frequency electronic and optoelectronic devices. Among these, AlGaN/GaN devices have received a lot of attention as they possess superior joint properties which include a large bandgap (i.e., 3.4 eV), high breakdown voltage, and high electron mobility.1−5 Due to its high thermal and long-term chemical stabilities, GaN may also offer an attractive and novel venture for developing molecular sensors where challenging physical and chemical environments are of concern. Sensors based on the Molecularly Controlled Semiconductor Resistor (MOCSER) were developed, when typically they were based on AlGaAs/GaAs. In these sensors the conventional gate system of a transistor, namely, the electrode and its dielectric oxide, were replaced by a molecular layer adsorbed directly on the semiconductor.6−8 This scheme ensures high sensitivity to molecular interactions on the surface, due to a direct contact between the adsorbed molecules and the semiconductor. In principle, the electronic and chemical properties of the adsorbed molecules define their interaction with the device’s surface states and modify the electrochemical potential on the surface. This is translated to the band-bending of the semiconductor and to deep impurity surface states that are either passivated or activated. We have demonstrated the application of these devices in various sensing fields, both in gas phase and in aqueous physiological environments.9−15 It was shown that similarly structured MOCSER devices made from AlGaN/GaN can exhibit molecular sensitivities toward various gases such as hydrogen, carbon dioxide, and ammonia gases,16−21 or even toward biorelated applications such as detection of DNA, antibodies, and living cells.22 © XXXX American Chemical Society
Received: August 19, 2015 Accepted: December 9, 2015
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DOI: 10.1021/acssensors.5b00047 ACS Sens. XXXX, XXX, XXX−XXX
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wire-bonded devices. In all experiments, a constant 0.5 V was applied between the source and drain leads. Exposure of the devices to the analytes was performed in a dark chamber with a constant flow of N2. The analytes used in this work were formic acid (HCOOH) and ammonium hydroxide (NH4OH). NH4OH was purchased from J.T. Baker and formic acid was purchased from Sigma-Aldrich. The different analytes were evaporated from a 1.5 mL tube which was placed in a 15 mL chamber at room temperature, and the vapors were carried by a constant N2 flow of 100 cc/min. This stream of nitrogen was combined with the main nitrogen flow and then introduced into the sensor chamber. Within the framework of this study, three sets of experiments were carried out: (1) response measurements to formic acid at fixed VG levels; (2) response to formic and ammonia by sweeping VG, at four different wavelength illuminations (0.03 μW LEDs 630, 587, 510, and 470 nm at a power density of 3.85 mW/m2); and (3) wavelength dependence measurements at fixed VG, in which the wavelength sweeping was done between 430 and 700 nm using a 150 W xenon lamp (Sciencetech Inc.) and a monochromator (iHR320). In the latter set, the intensity of the illumination was monitored using bolometers laser power sensor (OPHIR photonics) and the results were normalized accordingly. Contact Potential Difference (CPD) Surface and Photovoltage Spectroscopy (SPS) Measurements. Contact potential differences between samples and a gold grid reference electrode were measured in a Kelvin-probe configuration in a dry N2 environment. The CPD/SPS was measured after stabilizing in the dark. For SPS measurements, the samples were illuminated with a 600 W tungsten− halogen lamp and a monochromator at wavelengths varying between 400 and 900 nm.
Figure 1. (a) Schematic representation of the AlGaN/GaN based molecular controlled semiconductor resistor devices and the setup used in this work. A flow of nitrogen was used to transfer the gas molecules to the GaN-based MOCSER, while device illumination was done at varying wavelengths using LEDs or a monochromator with a xenon lamp. Images (b) and (c) illustrate the energetic band diagrams of the device when molecules of two opposite dipoles are adsorbed on the surface. The conductive 2D electron gas (2DEG) channel between the AlGaN and GaN layers is indicated by a filled gray area.
control the underlying band-bending, and thus the charge carrier density, yet on high enough voltage they can control the effective width of the conductive channel. The devices were coated with two similarly adsorbed 18-carbon-long alkylated monolayers bound to the GaN hydroxylated surfaces either via carboxylic acid or via phosphonic acid. Hence, the study aims at providing an insight into the sensing mechanism and the effect of illumination by visible light on the process.
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RESULTS Aiming at exploring the effect of visible light on the sensitivity of the GaN-based MOCSER, we examined devices coated with two types of molecular coatings, both carrying a saturated hydrocarbon chain of C18 but with different binding groups, namely, phosphonic acid (i.e., C18Phosp) and carboxylic acid (i.e., C18Carboxyl). Two analytes were detected, one is a good electron acceptor and the other an electron donor, formic acid and ammonia, respectively. The first has vapor pressure of about 40 mmHg while the second 7000 mmHg at 24 °C. By altering the potential applied on the two side gates we could modulate the effective dimensions of the conductive channel, and thus the device sensitivity. Figure 2 shows the response of the sensor to formic acid measured as a function of time when the devices were biased with a constant source-drain voltage (VSD) of 0.5 V and gate voltages (VG) varying from 2 to −6 V. Since the device conductive channel is n-type, the current through the devices drops as the negative gate potential increases (Figure 2a). Moreover, as the negative gate potential increases the sourcedrain current decreases upon exposure to the analyte. Interestingly, on positively applied gate voltages, a minor flip in the response was observed. Figure 2b summarizes the changes in the current, upon exposing the device to formic acid, as a percentage of change in the current, for bare or C18Carboxyl- and C18Phosp-coated devices. Among the three types of devices, C18Carboxyl coated devices exhibit the largest signals reaching up to 25% change in current. In the second set of experiments, the source-drain current was recorded while the gate voltage was changed and the devices exposed to vapors of formic acid or ammonium hydroxide (see Figure 3). In these experiments we have also examined the effect of illumination on the response of the devices to the analytes. Figure 3 shows a clear difference in response of the devices coated with the two molecules when exposed to the two types of analytes. While in the case of
MATERIALS AND METHODS
Device Fabrication. A schematic representation of the devices and the setup used in this work is given in Figure 1. AlGaN/GaN HEMT wafers were purchased from NTT-AT. The epitaxial structure was grown on sapphire substrate with the following layers (from the substrate up): i-GaN (1800 nm), i-AlGaN (20 nm), and i-GaN (2 nm). Devices were fabricated by photolithography using standard clean-room facilities. N-type Ohmic contacts were deposited using Ti/ Al/Ni/Au (20/1000/400/600 nm) and annealed at 900 °C. Device isolation was done by mesa etch using a BCl3/Cl2 ICP-RIE process. The width and length of the conducting channel was 8 and 12 μm, respectively. Gold lateral gates were deposited on top of the channel with a gate length of 6 μm and a spacing of 4 μm between the two gates. A 30 nm ALD-AlxOy coating was used as the dielectric layer below the gate. The detector active window opening, of about 20 μm2, was done by etching the AlxOy layer using buffered HF (6:1). Molecular Modification. Two organic monolayers were adsorbed on the devices according to previously reported procedures: stearic acid (C18Carboxyl) and 1-octadecanephosphonate (C18Phosp).23,24,26 All solvents used were reagent grade or better, purchased from Merck, Baker, or Bio-Lab. Stearic acid (C18Carboxyl) and n-octadecylphosphonic acid (C18Phosp) were purchased from Sigma-Aldrich and PCI Synthesis, respectively. All chemicals were used without further purification. Prior to molecular adsorption, the devices were sonicated for 10 s in hot acetone and ethanol, etched for 30 s in 6 M HCl, rinsed in water, and dried under a N2 stream. The samples were then treated with UV/ ozone oxidation (UVOCS) for 30 min, and placed immediately in the adsorption solution of 1 mM in toluene. The absorption reaction was carried under N2 in a dark desiccator for 19 or 65 h, for C18Phosp or C18Carboxyl, respectively. After adsorption, the samples were rinsed with toluene and dried with N2 stream. Monolayer quality was verified by FT-IR spectroscopy (SI1) and contact angle (109° for C18Phosp and 76° for C18Carboxyl). Electrical Measurements. Electrical measurements were performed using a dual-channel source measure unit (Keithley 2636A) on B
DOI: 10.1021/acssensors.5b00047 ACS Sens. XXXX, XXX, XXX−XXX
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ammonium hydroxide, positive change in currents was detected, in the case of formic acid we observed negative changes in current. In these measurements there is an optimum of the change in the current, maximum for ammonia or minimum for formic, at VG of about 4.5 V. In order to investigate the correlation between light illumination and the electrical response to the analytes, we illuminated the devices with four different LEDs that emit light with energies that are within the GaN band gap: 630 nm (1.97 eV), 587 nm (2.11 eV), 510 nm (2.43 eV), and 470 nm (2.63 eV). In all cases, illumination leads to a decrease in the device currents. Illumination with green or red light leads to a larger decrease in currents than illumination with orange or blue light; however, no simple correlation was found between the photons‘ energy and the response. A third set of measurements have cross-examined the influence of illumination at different wavelengths and the exposure to the different analytes on the response of the MOCSER devices (Figure 4). Devices were exposed first to the analytes in the dark and then illuminated while the vapors of the analytes were continuously flowing. The source-drain voltage was kept constant at 0.5 V and the gate voltage was varied from +2 to −6 V. The illumination wavelengths were scanned between 430 and 700 nm, at a step rate of 10 nm/min. The measurements were normalized to the current measured just before the illumination, I0, as indicated in the plot. While the current almost does not vary as a function of the gate voltage, upon exposure to ammonium hydroxide, large gate dependence was observed upon exposure to formic acid. Under illumination, all devices show a clear wavelength dependence pattern. In all cases the current in the devices increases upon illumination. This increase was found to be unaffected by the direction of the illumination sweep, namely, if the device was first exposed to 430 nm and the wavelength was swept to 700 nm or vice versa. Interestingly, from 610 nm until 700 nm the gradient in the current enhancement as a function of wavelength becomes larger. For gate voltages below −4 V, the response to the illuminations was less affected by the gate potential or by the analytes. The responses to ammonium hydroxide and formic acid are clearly different. The responses to illumination using different analytes follow the same trend. These results hint that the mechanisms, by which the analytes and the illuminations are affecting the current, are not the same. In order to understand the long-range contribution of the illumination to the sensing mechanism, surface photovoltage spectroscopy (SPS) measurements were conducted on all three types of surface coatings. In all cases, illumination at different
Figure 2. Device responses for vapors of formic acid at varying gate voltages (VG). (a) Typical ISD vs time plot measured for a C18Carboxyl-coated device. (b) Normalized change in current as a function of VG for bare GaN (black), GaN/C18Phosp (red), and GaN/ C18Carboxyl (blue). ΔI was measured as the current at 5 min after analyte exposure minus the current before the exposure (i.e., I0).
Figure 3. Device responses of GaN/C18Phosp and (a, c) GaN/ C18Carboxyl (b, d) MOCSER devices to ammonium hydroxide and formic acid while sweeping the gate voltage. The responses were recorded in darkness (black line) and at different illuminations: 630 nm (red line), 587 nm (orange line), 510 nm (green line), and 470 nm (blue line). The response of the device is defined as ΔI = Ianalyte − IN2. The gate sweep rate was set to be about 60 mV/s.
Figure 4. Device responses of GaN/C18Carboxyl coated MOCSER to (a) a plain carrier gas, N2, (b) ammonium hydroxide, and (c) formic acid at varying gate voltages (VG). ΔI was calculated as I − I0, where I0 indicates the dark point just before illumination. Illumination of the devices was done by sweeping the wavelength between 430 and 700 nm. C
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Figure 5. Schematic band diagrams of AlGaN/GaN illustrating the response mechanism to gate sweeping and illumination. (a) Sub-bandgap localized trapped states are located below the Fermi level when no gate bias is applied. (b) Sweeping at negative gates and introducing the analytes to the system decrease the Fermi level and influence the distribution of the charge carriers in the 2DEG. (c) By light illumination there is an enhancement of the electrons injected into the conduction band. The solid and hollow dots at the trapping states Et and near the Fermi energy level Ef represent the filled (electron) and empty (holes) trap states, respectively.
and 700 nm followed by full response to all examined wavelengths at further negative gate voltages. Hence, these results help in locating the resonating trap states’ energies. Beyond the effect of illumination this study explores several other properties of the hybrid device. Regarding the molecular coating of the device we have found that the electrical response to the two analytes, by the three different AlGaN/GaN MOCSERs, depends on the coating (Figures 2b and 3). While for the GaN/C18Carboxyl device we observed a large response to the analytes, in the other two, GaN/C18Phosp and Bare GaN devices, the changes in currents were smaller. A similar effect has been found for binding those molecules on GaAs based devices and it is attributed to the large charge density on the phosphonate that screens the interaction with the analyte.34 When the MOCSERs are coated with C18Carboxyl the CPD decreases. This means that the surface becomes more positively charged. Combining this finding with the fact that the device is a depleted n-type device means that the device should be more sensitive to negatively charged molecules, namely, more to formic acid than to ammonia, as indeed was observed. From the results seen mainly in Figure 2 and Figure 4, it is evident that the more negatively biased the side-gates are, and thus the narrower the conducting channel is, the bigger the responses to the formic acid. These two figures present consistent results on the effect of the gate on the response. However, Figure 3 is different due to the fast sweeping mode. Since the MOCSER device has by definition an exposed channel, it has a lot of surface states. These states are affected already when bias is applied between the source and drain, leading to a slow stabilization of the device currents (about 5 min). This phenomenon is known as the current-collapse. Hence when scanning the gate voltage fast enough, many transient states are still populated and have an additional effect beyond the steady state situation, as monitored in Figures 2 and 4. The combination of all the effects described above enables us to develop a set of identifying elements which can enhance the specificity of the device to a certain molecule analyte, reducing the false positive signatures.
wavelengths did not result in any measurable SPS signal (data not shown). Contact potential difference for bare, GaN/ C18Phosp, and GaN/C18Carboxyl were −0.73 eV, −0.82 eV, and −0.85 eV, respectively. These results indicate that the light does not affect the surface workfunction.
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DISCUSSION The effect of light on the GaN based devices poses a challenge, since one does not expect to observe such an effect for a wide band gap material. Naturally, the first assumption is that the sensitivity to illumination is related to surface states. Since the sensing of the MOCSER is related to manipulation of surface states, one would expect that if indeed the surface states are involved in the absorption then it will also affect the sensing. The current study demonstrates clearly that the response of the device to light and its molecular sensing are two independent complementary phenomena. The lack of SPS supports the conclusion that the common phenomenon in which illumination affects the band bending is not relevant for the present device. We suggest that deep states at the interface between the AlGaN and GaN layers are responsible to the visible light absorption (see Figure 5). These states could be affected by the adsorbed molecules. Exciting these states affects the conduction since it changes the electric potential near the conducting channel. In the devices we present here we differ between two operational modes: gate-sweeping mode which gives rise to sensing via transient states (as seen in Figure 3), and constant bias in which the devices are set to steady-state prior to sensing (as seen in Figure 4). These two modes enable an almost orthogonal sensing of the different elements. Specifically, Figure 3 shows that introduction of light at different wavelengths always reduces the change in currents upon exposure to the analytes. In the case of formic acid, this means increasing the signal, since the analyte by itself also does it and the addition of light enhances the analyte’s effect. However, in the case of ammonium, the signal almost disappears due to the light, since the light has the opposite effect on the current than the analyte. On the other hand, when applying a constant gate potential on the device and varying the illuminated wavelengths, the magnitude of the response to the varying wavelengths is analyte insensitive, as shown in Figure 4. Interestingly, the responses increase inversely to the photons’ energy. Based on Figure 4, it seems that the trap states near the 2DEG are shifted upon applied gate voltages that are greater than −2 V. This is observed first for illuminations between 610
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00047. D
DOI: 10.1021/acssensors.5b00047 ACS Sens. XXXX, XXX, XXX−XXX
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IR spectroscopy characteristics of the coated surfaces and electrical characteristics of the fully gated device versus side-gated devices (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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